Prior to 1960 most artifical feet were made from solid wood with a series of hinge or pivot points utilized to simulate natural foot movement. Upon the introduction of so-called one-piece SACH type foot construction which utilized a rigid core or keel portion surrounded by a molded flexible, polyurethane covering, many of the adverse problems of the prior constructions were improved. However, this construction also created a new set of problems, most of which related to the rigid core which consisted of wood or metal. Although many attempts have been made to improve this type of construction, most artifical feet manufactured up to the present use of wood core or keel since the metal keels were found to be even more unsatisfactory.
For example, the metal keels generally proved to be heavier than desirable and were even more difficult to bond well to the polyurethane outer cover surface than wooden keels. Insufficient bonding at the interface between the core and outer covering leads to early failures.
However, improved as it was over the much earlier constructions, the wood keel construction and molded flexible outer cover have been much less than satisfactory for many reasons.
First, wood cores must be hand crafted which fails to achieve a desirable level of uniformity and increases the cost of manufacture.
Second, inherently wood is subject to non-uniformity in grain structure, mineral streaks, etc., such that individual testing is required which further increases costs.
Third, it is very important to maintain the wood keel moisture free to ensure strength and to obtain at least reasonable bonding to the polyurethane outer surface. The tendency of wood to absorb moisture makes control of this variable difficult and costly.
Fourth, polyurethanes do not bond as well as desired to dissimilar materials, such as wood, and therefore even under ideal conditions, the bonding interface is subject to more than a reasonable number of failures which shortens useful life.
Further, since failures and long life are not the rule with prior art type artifical feet, a rather simple means of connection must be employed to permit easy replacement. Typically a bolt is inserted through the keel which is also attached to the artifical leg portion. Therefore as a wood keel expands and contracts with climatic changes and through the natural forces encountered during use, a loss of torque of the attaching bolt occurs. This results in dangerous as well as embarrassing rotation of the foot relative to the upper portion of the leg.
In spite of all of these problems, the wooden keel type construction still dominates the industry and many prior attempts to improve these drawbacks have failed or lead to problems of equal or more serious dimensions.
For example, one suggested solution was substitution of a metal plate, aluminum, for example, for the entire keel with the flexible polyurethane molded around it. However, since this plate is subject to relatively high bending movements, thickness was required which resulted in added weight. Increased weight at this point so far from the knee center of an artifical leg results in an increased moment about the knee hinging point and causes an undesirable stress on control mechanisms which regulate the flexion and extension of the shin portion relative to the thigh. In addition, added weight in this portion of the foot causes an undesirable increase in terminal impact upon full extension of the shin.
A further disadvantage to this construction, which is disclosed in U.S. Pat. No 3,890,650, is that the void above the metal plate must be filled with a material of sufficient strength to cope with the high forces generated at the attaching point of the foot to the artifical leg. This again adds more weight and increases the detrimental results mentioned above. Also, it should be noted that this construction requires a surface bond between a non-porous material, the metal plate, to the flexible urethane. Such a bond is notoriously subject to failure and results in a short useful life of the product.
Other examples of attempts to solve the problems encountered with the manufacture of artifical feet are represented by U.S. Pat. Nos. 3,098,239; 3,484,871; 3,766,569; 3,833,941; 3,920,610 and 3,874,004.